cortex: Cortex can read rules but doesn't activate them

Description

I’m running 1.4.0 using the binary from GitHub and I have ruler configured to send alerts to my own cluster of Alertmanager.

For a moment I saw the alerts in my Alertmanager Web UI, but shortly after they disappeared.

Config

My ruler section of the config looks like this:

ruler:
  external_url: 'https://alerts.example.org/'
  alertmanager_url: 'http://localhost:9093/'
  enable_alertmanager_v2: true
  rule_path: '/var/tmp/cortex/rules'
  enable_api: true
  storage:
    type: local
    local:
      directory: '/etc/cortex/rules'

My rules are located in /etc/cortex/rules/fale since I use auth_enabled: false.

Debugging

I can see the rules are located in the right place because I can look them up using the /api/v1/rules call:

 > curl -s 'http://localhost:9092/api/v1/rules' | head
instance.yml:
    - name: instance
      rules:
        - alert: InstanceDown
          expr: up == 0
          for: 5m
          annotations:
            current_value: '{{ $value }}'
            description: '{{ $labels.instance }} of job {{ $labels.job }} has been down for more than 5 minutes.'
            summary: Instance {{ $labels.instance }} down

But, when I try to use the /prometheus/api/v1/rules path I get nothing:

 > curl -s 'http://localhost:9092/prometheus/api/v1/rules' -H 'X-Scope-OrgID: fake' | jq .
{
  "status": "success",
  "data": {
    "groups": []
  },
  "errorType": "",
  "error": ""
}

Even though just minutes ago I saw the rules displayed here. As well as the alerts generated by the rules. But now there’s nothing there:

 > curl -s 'http://localhost:9092/prometheus/api/v1/alerts' -H 'X-Scope-OrgID: fake' | jq .
{
  "status": "success",
  "data": {
    "alerts": []
  },
  "errorType": "",
  "error": ""
}

I’m confused as to what caused them to disappear. Restarting Cortex nodes doesn’t fix the issue.

Questions

• My understanding is that ruler.rule_path is the place where Cortex checks for rule files. Correct?
• My understanding is that ruler.storage.local.directory configures a temporary location for rule files. Correct?
• Why can the rules be loaded from ruler.rule_path but are not available via /prometheus/api/v1/rules?